Please forgive me if this has been asked before. I am considering changing from my sony 505 to a Kindle. I am currently using Calibre to upload books to my Sony. Can Calibre convert my books for the Kindle or am I going to lose all my books on the Sony. Is there another simple way of keeping my old book list?
I am "computer challenged" and need answers in very simple terms, please.

For books in your calibre library that are not locked with DRM, all you will need to do to make a book readable on your Kindle is to right-click on the book and select [convert], and in the upper-right drop-down of the conversion window, select mobi as the output format.
I would suggest you do individual books at a time so you can check the results and see if/how you want to tweak the conversion settings, e.g., margins, spaces between paragraphs, etc. Usually the defaults work just fine, though.

For books that are locked with DRM (which would be most likely for books published by the big name publishers), you must first strip the DRM, and then convert to .mobi.

Since you have calibre, you can do this very easily (drag and drop with a plugin), and since you paid for the book and want to strip it for personal use, I think most people would do so with no qualms.

But it is a personal choice, and since the laws vary by country, we aren't allowed to give you the instructions for exactly how to do it. You would need to google Apprentice Alf.